There are three types of DNA testing in genetic genealogy: autosomal DNA, Y-DNA, and mitochondrial DNA.
Autospmal DNA refers to the 22 pairs of non-sex chromosomes in the nucleus that are inherited equally from both parents. The amount of DNA shared by two people (measured in centimorgans, cM) can be used to help estimate the relationship between the people. E.g., first cousins share on average 850 cM; third cousins share on average 53 cM.
The Y-chromosome, one of the two sex chromosomes, is found only in men, inherited from their fathers. It is helpful in breaking down brick walls in the paternal line.
Mitrochondrial DNA Is passed down from a mother to her male and female children. It may be helpful in determining whether two people are related on the maternal line.
The X-chromosome is the other sex chromosome. Women have two X-chromosomes, one inherited from her mother and one from her father. Men have one X and one Y, the X inherited from his mother and the Y from his father. Since a son doesn't receive any X-DNA from his father, an X-DNA match reduces the set of potential common ancestors. Some companies report X-DNA results with the autosomal test results.
Here is a summary of tests provided by each testing site, including links to each site.